U.S. President Donald Trump on Thursday signed a 25 percent tariff on steel imports that is certain to add pressure on South Korean exports.

Trump signed two proclamations -- one for steel and another for a 10 percent tariff on aluminum imports -- to address what the administration has called a threat to national security.

South Korea was the third largest exporter of steel products to the U.S. last year, after Canada and Brazil. The new action comes after the U.S. introduced safeguard measures against Korean washers and solar panels earlier this year.

This illustration uses an EPA file photo of U.S. President Donald Trump. (Yonhap)

The tariffs will take effect in 15 days.

Mexico and Canada were the only countries to be exempted as negotiations to amend the North American Free Trade Agreement with the U.S. are currently under way.

The proclamations include provisions to allow other countries to discuss alternative ways to address the threat posed by the metal exports, a senior administration official told reporters.

Trump announced the tariffs last week following recommendations from the U.S. Department of Commerce.

"We have to protect and build our Steel and Aluminum Industries while at the same time showing great flexibility and cooperation toward those that are real friends and treat us fairly on both trade and the military," he tweeted earlier Thursday.